Guillemette is the godmother of journalist Tristane Banon, 32, who has lodged a formal complaint against the former International Monetary Fund chief awaiting his next court appearance in New York on a sex assault case there.
Detectives from the violent crimes squad interviewed his ex-wife on Friday, a source said, confirming a report on the Le Point news magazine website.
Banon's mother, Anne Mansouret, has said recently in several press interviews that she had contacted Strauss-Kahn's ex-wife shortly after the alleged sexual assault on her daughter.
Banon accuses Strauss-Kahn of attacking her during an interview in February 2003 in a central Paris flat. She was herself questioned by police on July 11.
Police also interviewed Friday an online journalist who had worked with Banon, the source said.
Prosecutors will later decide whether to charge the 62-year-old Socialist politician, who was at one time expected to challenge Nicolas Sarkozy in next year's presidential election.
Strauss-Kahn faces criminal charges in New York for allegedly trying to rape a 32-year-old hotel maid in May, which he denies.
He has also dismissed Banon's claims and his lawyers said they have filed a lawsuit for defamation against the young writer, who in a 2007 television interview accused Strauss-Kahn of trying to force himself on her "like a rutting chimpanzee."
Strauss-Kahn's name was bleeped out but Banon has since confirmed in a separate interview she was referring to him.
A source to close to the Banon investigation said Monday that Francois Hollande, the former head of the Socialist Party and a candidate for president in the party's primary this year, is expected to be questioned by police soon.
The Figaro newspaper indicated the interview would take place in early September.
Both Banon and her mother have said Hollande was told about the young woman's claims against Strauss-Kahn, but he has said he did not know in detail what allegedly happened.